1. Field of the Invention
This invention relates to electrical devices in which an electrode is in contact with a conductive polymer composition.
2. Statement of the Prior Art
Conductive polymer compositions are well known. They comprise organic polymers having dispersed therein a finely divided conductive filler, for example carbon black or a particulate metal. Some such compositions exhibit so-called PTC (Positive Temperature Coefficient) behavior, i.e. they exhibit a rapid increase in electrical resistance over a particular temperature range. These conductive polymer compositions are useful in electrical devices in which the composition is in contact with an electrode, usually of metal. Devices of this kind are usually manufactured by methods comprising extruding or moulding the molten polymer composition around or against the electrode or electrodes. In the known methods, the electrode is not heated prior to contact with the polymer composition or is heated only to a limited extent, for example to a temperature well below the melting point of the composition. Well known examples of such devices are flexible strip heaters which comprise a generally ribbon-shaped core (i.e. a core whose cross-section is generally rectangular or dumbell-shaped) of the conductive polymer composition, a pair of longitudinally extending electrodes, generally of stranded wire, embedded in the core near the edges thereof, and an outer layer of a protective and insulating composition. Particularly useful heaters are those in which the composition exhibits PTC behavior, and which are therefore self-regulating. In the preparation of such heaters in which the composition contains less than 15% of carbon black, the prior art has taught that it is necessary, in order to obtain a sufficiently low resistivity, to anneal the heater for a time such that EQU 2L+5 log.sub.10 R.ltoreq.45
where L is the percent by weight of carbon and R is the resistivity in ohm.cm. For further details of known PTC compositions and devices comprising them, reference may be made to U.S. Pat. Nos. 2,978,665, 3,243,753, 3,412,358, 3,591,526, 3,793,716, 3,823,217, and 3,914,363, the disclosures of which are hereby incorporated by reference. For details of recent developments in this field, reference may be commonly assigned made to U.S. patent application Ser. Nos. 601,638 (now U.S. Pat. No. 4,177,376), 601,427 (now U.S. Pat. No. 4,017,715), 601,549 (now abandoned), and 601,344 (now U.S. Pat. No. 4,085,286) (all filed Aug. 4, 1975), 638,440 (now abandoned in favor of continuation-in-part application Ser. No. 775,882 issued as U.S. Pat. No. 4,177,446) and 638,687 (now abandoned in favor of continuation-in-part application Ser. No. 786,835 issued as U.S. Pat. No. 4,135,587) (both filed Dec. 8, 1975), the disclosures of which are hereby incorporated by reference.
A disadvantage which arises with devices of this type, and in particular with strip heaters, is that the longer they are in service, the higher is their resistance and the lower is their power output, particularly when they are subject to thermal cycling.
It is known that variations, from device to device, of the contact resistance between electrodes and carbon-black-filled rubbers is an obstacle to comparison of the electrical characteristics of such devices and to the accurage measurement of the resistivity of such rubbers, particularly at high resistivities and low voltages; and it has been suggested that the same is true of other conductive polymer compositions. Various methods have been suggested for reducing the contact resistance between carbon-black-filled rubbers and test electrodes placed in contact therewith. The preferred method is to vulcanise the rubber while it is in contact with a brass electrode. Other methods include copper-plating, vaccum-coating with gold, and the use of colloidal solutions of graphite between the electrode and the test piece. For details, reference should be made to Chapter 2 of "Conductive Rubbers and Plastics" by R. H. Norman, published by Applied Science Publishers (1970), from which it will be clear that the factors which govern the size of such contact resistance are not well understood. So far as we know, however, it has never been suggested that the size of the initial contact resistance is in any way connected with the changes in resistance which take place with time in devices which comprise an electrode in contact with a conductive polymer composition, e.g. strip heaters.